Turbocharger including cast titanium compressor wheel

ABSTRACT

An air boost device such as a turbocharger, wherein the compressor wheel thereof is re-designed to permit die inserts ( 20 ), which occupy the air passage and define the blades ( 4, 5 ) during a process of forming a wax pattern ( 21 ) of a compressor wheel, to be pulled without being impeded by the blades. This modified blade design enables the automated production of wax patterns ( 21 ) using simplified tooling. The compressor wheel improves low cycle fatigue, withstands high temperatures and temperature changes, and permits operation at high boost pressure ratio while, on the other hand, having low weight, low inertial drag, and high responsiveness.

FIELD OF THE INVENTION

[0001] The present invention concerns a titanium compressor wheel foruse in an air boost device, capable of operating at high RPM withacceptable aerodynamic performance, yet capable of being producedeconomically by an investment casting process.

DESCRIPTION OF THE RELATED ART

[0002] Air boost devices (turbochargers, superchargers, electriccompressors, etc.) are used to increase combustion air throughput anddensity, thereby increasing power and responsiveness of internalcombustion engines. The design and function of turbochargers aredescribed in detail in the prior art, for example, U.S. Pat. Nos.4,705,463, 5,399,064, and 6,164,931, the disclosures of which areincorporated herein by reference.

[0003] The blades of a compressor wheel have a highly complex shape, for(a) drawing air in axially, (b) accelerating it centrifugally, and (c)discharging air radially outward at elevated pressure into thevolute-shaped chamber of a compressor housing. In order to accomplishthese three distinct functions with maximum efficiently and minimumturbulence, the blades can be said to have three separate regions.

[0004] First, the leading edge of the blade can be described as a sharppitch helix, adapted for scooping air in and moving air axially.Considering only the leading edge of the blade, the cantilevered oroutboard tip travels faster (MPS) than the part closest to the hub, andis generally provided with an even greater pitch angle than the partclosest to the hub (see FIG. 1). Thus, the angle of attack of theleading edge of the blade undergoes a twist from lower pitch near thehub to a higher pitch at the outer tip of the leading edge. Further, theleading edge of the blade generally is bowed, and is not planar. Furtheryet, the leading edge of the blade generally has a “dip” near the huband a “rise” or convexity along the outer third of the blade tip. Thesedesign features are all designed to enhance the function of drawing airin axially.

[0005] Next, in the second region of the blades, the blades are curvedin a manner to change the direction of the airflow from axial to radial,and at the same time to rapidly spin the air centrifugally andaccelerate the air to a high velocity, so that when diffused in a volutechamber after leaving the impeller the energy is recovered in the formof increased pressure. Air is trapped in airflow channels definedbetween the blades, as well as between the inner wall of the compressorwheel housing and the radially enlarged disc-like portion of the hubwhich defines a floor space, the housing-floor spacing narrowing in thedirection of air flow.

[0006] Finally, in the third region, the blades terminate in a trailingedge, which is designed for propelling air radially out of thecompressor wheel. The design of this blade trailing edge is generallycomplex, provided with (a) a pitch, (b) an angle offset from radial,and/or (c) a back taper or back sweep (which, together with the forwardsweep at the leading edge, provides the blade with an overall “S”shape). Air expelled in this way has not only high flow, but also highpressure.

[0007] Recently, tighter regulation of engine exhaust emissions has ledto an interest in even higher pressure ratio boosting devices. However,current compressor wheels are not capable of withstanding repeatedexposure to higher pressure ratios (>3.8). While aluminum is a materialof choice for compressor wheels due to low weight and low cost, thetemperature at the blade tips, and the stresses due to increasedcentrifugal forces at high RPM, exceed the capability of conventionallyemployed aluminum alloys. Refinements have been made to aluminumcompressor wheels, but due to the inherent limited strength of aluminum,no further significant improvements can be expected. Accordingly, highpressure ratio boost devices have been found in practice to have shortlife, to be associated with high maintenance cost, and thus have toohigh a product life cost for widespread acceptance.

[0008] Titanium, known for high strength and low weight, might at firstseem to be a suitable next generation material. Large titaniumcompressor wheels have in fact long been used in turbojet engines andjet engines from the B-52B/RB-52B to the F-22. However, titanium is oneof the most difficult metals to work with, and currently the cost ofproduction associated with titanium compressor wheels is so high as tolimit wide spread employment of titanium.

[0009] There are presently no known cost-effective manufacturingtechniques for manufacturing automobile or truck industry scale titaniumcompressor wheels. The automotive industry is driven by economics. Whilethere is a need for a high performance compressor wheel, it must becapable of being manufactured at reasonable cost.

[0010] One example of a patent teaching casting of compressor wheels isU.S. Pat. No. 4,556,528 (Gersch et al) entitled “Method and Device forCasting of Fragile and Complex Shapes”. This patent illustrates thecomplex design of compressor wheels (as discussed in detail above), andthe complex process involved in forming a resilient pattern forsubsequent use in forming molds. More specifically, Gersch et al teach aprocess involving placing a solid positive resilient master pattern ofan impeller into a suitable flask, pouring a flexible and resilientmaterial, such as silastic or platinum rubber material, over the masterpattern, curing, and withdrawing the solid master pattern of theimpeller from the flexible material to form a flexible mold with areverse or negative cavity of the master pattern. A flexible andresilient curable material is then poured into the cavity of the reversemold. After the flexible and resilient material cures to form a positiveflexible pattern of the impeller, it is removed from the flexiblenegative mold. The flexible positive pattern is then placed in an opentop metal flask, and foundry plaster is poured into the flask. After theplaster has set up, the positive flexible pattern is removed from theplaster, leaving a negative plaster mold. A non-ferrous molten material(e.g., aluminum) is poured into the plaster mold. After the non-ferrousmolten material solidifies and cools, the plaster is destroyed andremoved to produce a positive non-ferrous reproduction of the originalpart.

[0011] While the Gersch et al process is effective for forming castaluminum compressor wheels, it is limited to non-ferrous or lowertemperature or minimally reactive casting materials and cannot be usedfor producing parts of high temperature casting materials such asferrous metals and titanium. Titanium, being highly reactive, requires aceramic shell.

[0012] U.S. Pat. No. 6,019,927 (Galliger) entitled “Method of Casting aComplex Metal Part” teaches a method for casting a titanium gas turbineimpeller which, though different in shape from a compressor wheel, doeshave a complex geometry with walls or blades defining undercut spaces. Aflexible and resilient positive pattern is made, and the pattern isdipped into a ceramic molding media capable of drying and hardening. Thepattern is removed from the media to form a ceramic layer on theflexible pattern, and the layer is coated with sand and air-dried toform a ceramic layer. The dipping, sanding and drying operations arerepeated several times to form a multilayer ceramic shell. The flexiblewall pattern is removed from the shell, by partially collapsing withsuction if necessary, to form a first ceramic shell mold with a negativecavity defining the part. A second ceramic shell mold is formed on thefirst shell mold to define the back of the part and a pour passage, andthe combined shell molds are fired in a kiln. A high temperature castingmaterial is poured into the shell molds, and after the casting materialsolidifies, the shell molds are removed by breaking.

[0013] It is apparent that the Galliger gas turbine flexible pattern is(a) collapsible and (b) is intended for manufacturing large-dimensiongas turbine impellers for jet or turbojet engines. This technique is notsuitable for mass-production of automobile scale compressor wheels withthin blades, using a non-collapsing pattern. Galliger does not teach amethod which could be adapted to in the automotive industry.

[0014] In addition to the above “rubber pattern” technique for formingcasting molds, there is a well-known process referred to as “investmentcasting” which can be used for making compressor wheels and whichinvolves:

[0015] (1) making a wax pattern of a hub with cantilevered airfoils,

[0016] (2) casting a refractory mass about the wax pattern,

[0017] (3) removing the wax by solvent or thermal means, to form acasting mold,

[0018] (4) pouring and solidifying the casting, and

[0019] (5) removing the mold materials.

[0020] There are however significant problems associated with theinitial step of forming the compressor wheel wax pattern. Whenever a dieis used to cast the wax pattern, the casting die must be opened torelease the product. Herein, the several parts of the die (die inserts)must each be retracted, generally only in a straight (radial) line.

[0021] As discussed above, the blades of a compressor wheel have acomplex shape. The complex geometry of the compressor wheel, withundercut recesses and/or back tapers created by the twist of theindividual air foils with compound curves, not to mention dips and humpsalong the leading edge of the blade, impedes the withdrawal of dieinserts.

[0022] In order to side-step these complexities, it has been known tofashion separate molds for each of the wax blades and for the wax hub.The separate wax blades and hub can then be assembled and fused to forma wax compressor wheel pattern. However, it is difficult to assemble acompressor pattern from separate wax parts with the required degree ofprecision—including coplanerism of airfoils, proper angle of attack ortwist, and equal spacing. Further, stresses are encountered duringassembling lead to distortion after removal from the assembly fixture.Finally, this is a labor intensive and thus expensive process. Thistechnique cannot be employed on an industrial scale.

[0023] Certainly, titanium compressor wheels would seem desirable overaluminum or steel compressor wheels. Titanium is strong andlight-weight, and thus lends itself to producing thin, light-weightcompressor wheels wchich can be driven at high RPM without over-stressdue to centrifugal forces.

[0024] However, as discussed above, titanium is one of the mostdifficult materials to work with, resulting in a prohibitively high costof manufacturing compressor wheels. This manufacturing cost preventstheir wide-spread employment. No new technology will be adoptedindustrially unless accompanied by a cost benefit.

[0025] There is thus a need for a simple and economical method for massproducing titanium compressor wheels, and for the low-cost titaniumcompressor wheels produced thereby. The method must be capable ofreliably and reproducibly producing compressor wheels, without sufferingfrom the prior art problems of dimensional or structural imperfections,particularly in the thin blades.

SUMMARY OF THE INVENTION

[0026] The present invention addressed the problem of whether it wouldbe possible to design a titanium compressor wheel for boosting airpressure and throughput to an internal combustion engine and satisfyingthe following two (seemingly contradictory) requirements:

[0027] aerodynamically: the aerodynamic efficiency, when operating atthe high RPM at which titanium compressor wheels are capable ofoperating, must be comparable to the efficiency of the complexstate-of-the-art compressor wheel designs, and

[0028] manufacturability: the compressor wheels must be capable of beingmass produced in a manner that is more efficient than the conventionallyemployed methods described above.

[0029] The problem was solved by the present inventors in a surprisingmanner. Simply stated, the present inventors approached this problem bystanding it on it's head. Traditionally, a manufacturing process beginsby designing a product, and then devising a processes for making thatproduct. Most compressor wheels are designed for optimum aerodynamicefficiency, and thus have narrow blade spacing and complex leading andtrailing edge design (excess rake, undercutting and backsweep, complexbowing and leading edge hump and dip).

[0030] The present invention was surprisingly made by departing from theconventional engineering approach and by looking first not at the endproduct, but rather at the various processes for producing the waxpattern. The inventors then designed various compressor wheels on thebasis of “pullability”—ability to be manufactured using die insertswhich are pullable—and then tested the operational properties of variouscompressor wheels produced from these simplified patterns at high RPM,with repeated load cycles, and for long periods of time (to simulatelong use in practical environment). The result was a simplifiedcompressor wheel design which (a) lends itself to economical productionby casting of titanium, and (b) at high RPM has an entirely satisfactoryaerodynamic performance.

[0031] More specifically, the invention provides a titanium compressorwheel with a simplified blade design, which will aerodynamically have adegree of efficiency comparable to that of a complex compressor wheelblade design, and yet which, form a manufacturing aspect, can beproduced economically in an investment casting process (lost waxprocess) using a wax pattern easily producible at low cost from anautomated (and “pullable”) die.

[0032] As a result of this discovery, the economic equation has shiftedfor the first time in favor of the titanium compressor wheel for generalautomotive technology.

[0033] Accordingly, in a first embodiment, the invention concerns acompressor wheel of simplified blade design, such that:

[0034] a wax pattern can be formed in a die consisting of one or moredie inserts per compressor wheel air passage (i.e., the space betweenthe blades), and preferably two die inserts per air passage, and

[0035] the die inserts can automatically be extracted radially or alongsome compound curve or axis in order to expose the wax pattern for easyremoval.

[0036] The compressor wheel blades may have curvature, and may be of anydesign so long as the blade leading edges have no dips and no humps, andthe blades have no undercut recesses and/or back tapers created by thetwist of the individual air foils with compound curves of a magnitudewhich would prevent extracting the die inserts radially or along somecurve or arc in a simple manner.

[0037] In simplest form, the wax mold is produced from a die having onedie insert corresponding to each air passage. This is possible where theblades are designed to permit pulling of simple die inserts (i.e., onedie insert per air passage). However, as discussed below, teach die canbe comprised of two or more die inserts, with two inserts per airpassage being preferred for reasons of economy.

[0038] In a more advanced form, the blades are designed with some degreeof rake or backsweep or curvature, but only to the extent that two ormore, preferably two inserts, per air passage can be easilyautomatically extracted. Such an arrangement, though slightly increasingthe cost and complexity of the wax mold tooling, would permitmanufacture of wax molds, and thus compressor wheels, with greatercomplexity of shape. In the case of two inserts per air passage, thepull direction would not necessarily be the same for each member of thepair of inserts. The one die insert, defining one area of the airpassage between two blades, may be pulled radially with a slight forwardtilt, while a second die insert, defining the rest of the passage, maybe pulled along a slight arc due to the slight backsweep of the blade.This embodiment is referred to as a “compound die insert” embodiment.One way of describing pullability is that the blade surfaces are notconvex. That is, a positive draft exists along the pull axis.

[0039] Once the wax pattern is formed, the titanium investment castingprocess continues in the conventional manner.

[0040] The invention further concerns an economical method for operatingan internal combustion engine, comprising providing said engine with aneasily manufactured, long-life titanium compressor wheel and driving thetitanium compressor wheel at high RPM for increasing combustion airthroughput and density and reducing emissions.

[0041] The titanium compressor wheel of the present invention has adesign lending itself to being produced in a simplified, highlyautomated process.

[0042] The foregoing has outlined rather broadly the more pertinent andimportant features of the present invention in order that the detaileddescription of the invention that follows may be better understood, andso that the present contribution to the art can be more fullyappreciated. Additional features of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other compressor wheels for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent structures donot depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] For a fuller understanding of the nature and objects of thepresent invention reference should be made by the following detaileddescription taken in with the accompanying drawings in which:

[0044]FIG. 1 shows a compressor wheel of prior art design in elevatedperspective view;

[0045]FIG. 2 shows, in comparison to FIG. 1, a compressor wheel designedin accordance with the present invention, in elevated perspective view;

[0046]FIG. 3 shows a partial compressor wheel of prior art design inside profile view;

[0047]FIG. 4 shows, in comparison to FIG. 3, a partial compressor wheeldesigned in accordance with the present invention, in side profile view;

[0048]FIG. 5 shows an enlarged partial section of a compressor wheel ofprior art design in elevated perspective view;

[0049]FIG. 6 shows, in comparison to FIG. 5, an enlarged partial sectionof a compressor wheel designed in accordance with the present invention,in elevated perspective view;

[0050]FIG. 7 shows a simplified section, perpendicular to the rotationaxis of the compressor wheel, with die inserts defining the hub andblades of a compressor wheel;

[0051]FIG. 8 corresponds to FIG. 7 and shows a top view onto acompressor wheel sectioned perpendicular to the rotation axis at aboutthe center of the hub;

[0052]FIGS. 9 and 10 show a simplified arrangement for extracting a diealong a simple curve;

[0053]FIG. 11 shows a compressor wheel according to the invention, withslightly backswept trailing edge, for production using compound dieinserts.

DETAILED DESCRIPTION OF THE INVENTION

[0054] One major aspect of the present invention is based on anadjustment of an aerodynamically acceptable design or blade geometry soas to make a wax pattern, from which the cast titanium compressor wheelis produced, initially producible in an automatic die as a unitized,complete shape. The invention provides a simplified blade design which(a) allows production of wax patterns using simplified tooling and (b)is aerodynamically effective. This modified blade design is at the rootof a simple and economical method for manufacturing cast titaniumcompressor wheels.

[0055] The invention provides for the first time a process by whichtitanium compressor wheels can be mass produced by a simple, low cost,economical process. In the following the invention will first bedescribed using simple die inserts, i.e., one die insert per airpassage, after which an embodiment having compound die inserts, i.e.,two or more die inserts per air passage, will be described.

[0056] The term “titanium compressor wheel” is used herein to refer to acompressor wheel comprised predominantly of titanium. One example of asuitable titanium alloy consists of 90% titanium, 6% aluminum, and 4%vanadium. This is often simply referred to in the art as titanium, butis more accurately a “titanium alloy”, and these terms are usedinterchangeably herein.

[0057] As the starting point for understanding the present invention, itmust be understood that the shape, contours and curvature of the bladesare modified to provide a design which, on the one hand, providesaerodynamically acceptable characteristics at high RPM, and on the otherhand, makes it possible to produce a wax pattern economically using anautomatic compound die. That is, it is central to the invention that dieinserts used to define the air passages during casting of the waxpattern are “pullable”, i.e., can be withdrawn radially or along acurvature. In order to make the die inserts retractable, the followingaspects were taken into consideration:

[0058] the compressor wheel must have adequate blade spacing;

[0059] the compressor wheel may not exhibit excess rake and/or backsweepof the blade leading edge or trailing edge,

[0060] there may not be excessive twist in the blades,

[0061] there may be no dips or humps along the leading edge of the bladewhich would prevent pulling of the die inserts,

[0062] there may not be excessive bowing of the blade, and

[0063] the die inserts used in forming the wax pattern must beextractable along a straight line or a simple curve.

[0064] Once the wax pattern satisfying the above requirements has beenproduced, the remainder of the casting technique can be traditionalinvestment casting, with modifications as known in the art for castingtitanium. A wax pattern is dipped into a ceramic slurry multiple times.After a drying process the shell is “de-waxed” and hardened by firing.The next step involves filling the mold with molten metal. Moltentitanium is very reactive and requires a special ceramic shell materialwith no available oxygen. Pours are also preferably done in a hardvacuum. Some foundries use centrifugal casting to fill the mold. Mostuse gravity pouring with complex gating to achieve sound castings. Aftercool-down, the shell is broken and removed, and the casting is givenspecial processing to remove the mold-metal reaction layer, usually bychemical milling.

[0065] Some densification by HIP (hot isostatic pressing) may be neededif the process otherwise leaves excessive internal voids.

[0066] The invention will now be described in greater detail by way ofcomparing the compressor wheel of the invention to a compressor wheel ofthe prior art, for which reference is made to the figures.

[0067]FIGS. 1 and 3 show a prior art compressor wheel 1, comprising anannular hub 2 which extends radially outward at the base part to form abase 3. The transition from hub to base may be curved (fluted) or may beangled. A series of evenly spaced thin-walled full blades 4 and“splitter” blades 5 are form an integral part of the compressor wheel.Splitter blades differ from full blades mainly in that their leadingedge begins further axially downstream as compared to the full blades.The compressor wheel is located in a compressor housing, with the outerfree edges of the blades passing close to the inner wall of thecompressor housing. As air is drawn into the compressor inlet, passesthrough the air channels of the rapidly rotating compressor wheel, andis thrown (centrifugally) outwards along the base of the compressorwheel into an annular volute chamber, and this compressed air is thenconveyed to the engine intake. It is readily apparent that the complexgeometry of the compressor wheel, with dips 6 and humps 7 along theblade leading edge, undercut recesses 9 created by the twist of theindividual air foils with compound curves, and rake or back tapers (backsweep) 8 at the blade trailing edge, would make it impossible to castsuch a shape in one piece in an automatic process, since the geometrywould impede the withdrawal of die inserts or mold members.

[0068]FIGS. 2 and 4, in comparison, show a compressor wheel according tothe present invention, designed beginning foremost with the idea ofmaking die inserts easily retractable, and thus taking intoconsideration the interrelated concepts of adequate blade spacing,absence of excess rake and/or backsweep of the blade leading edge andtrailing edge, absence of dips or humps along the leading edge, andextractability of die inserts along a straight line or a simple curve.Simply stated, the main characterizing feature of the present inventionis the absence of blade features which would prevent “pullability” ofdie inserts.

[0069] These design considerations result, as seen in FIGS. 2 and 4, ina compressor wheel 11 (the wax pattern being identical in shape to thefinal titanium product, the figures could be seen as showing either thewax pattern or the cast titanium compressor wheel) with a hub 12 havinga hub base 13, and a series of evenly spaced thin walled full blades 14and “splitter” blades 15 cast as an integral part of the compressorwheel.

[0070] It can be seen that the leading edge 17 of the blades areessentially straight, having no dips or humps which would impede radialextraction of die inserts. That is, there may be a slight rounding up 18(i.e., continuation of the blade along the blade pitch) where the bladejoins the hub, but this curvature does not interfere with pullability ofdie inserts.

[0071] It can be seen that the blade spacing is wide enough and that anyrake and/or backsweep of the blades is not so great as to impedeextraction of the inserts along a straight line or a simple curve.

[0072] Trailing edge 16 of the blade 14 may in one design extendrelatively radially outward from the center of the hub (the hub axis)or, more preferably, may extend along an imaginary line from a point onthe outer edge of the hub disk to a point on the outer (leading)circumference of the hub shaft. The trailing edge of the blade, viewedfrom the side of the compressor wheel may be oriented parallel to thehub axis, but is preferably cantilevered beyond the base of the hub andextends beyond the base triangularly, as shown in FIG. 2, and isinclined with a pitch which may be the same as the rest of the blade, ormay be increased. Finally, as shown in FIG. 11, the blade may have asmall amount of backsweep (which, when viewed with the forward sweep ofthe leading edge, produced a slight “S” shape) but the area of the bladenear the trailing edge is preferably relatively planar.

[0073] In a basic embodiment, the compressor wheel has from 8 to 12 fullblades and no splitter blades. In a preferred embodiment, the compressorwheel has from 4 to 8, preferably 6, full blades and an equal number ofsplitter blades.

[0074]FIG. 3 shows a partial compressor wheel of prior art design inside profile view, with the blade leading edge exhibiting a dip 6 and ahump 7 producing a shape which would interfere with radial extraction ofdie inserts.

[0075]FIG. 4 shows a partial compressor wheel similarly dimensioned tothe wheel of FIG. 3, but as can be seen, with a substantially straightshoulder of the blade from neck 18 to tip 19.

[0076]FIG. 5 shows an enlarged partial section of a compressor wheel ofa prior art design in elevated perspective view, illustrating dip 6,hump 7, and bowing and curvature of the leading edge. It can also beseen that the “twist” (difference in pitch along the leading edge), inaddition to the curvature, would make it impossible to radially extracta die insert.

[0077]FIG. 6 shows an enlarged partial section of a partial compressorwheel according to the invention, similarly dimensioned to FIG. 5, butdesigned in accordance with the present invention, showing a straightleading edge 19 and an absence of any degree of twist and curvaturewhich would prevent pulling of die inserts.

[0078] Obviously, the above dimensions refer equally to the wax patternand the finished compressor wheel. The wax pattern differs from thefinal product mainly in that a wax funnel is included. This produces inthe ceramic mold void a funnel into which molten metal is poured duringcasting. Any excess metal remaining in this funnel area after casting isremoved from the final product, usually by machining.

[0079] In FIG. 7 the tool or die for forming the wax form is shown inclosed condition, in sectional view along section line 8 shown in FIG.6, and simplified (omitting mechanical extraction means, etc.) forbetter understanding of the essential feature of the invention,revealing a cross section through a compressor wheel shaped mold. Themold defines a hub cavity and a number of inserts 20 that occupy the airpassages between the blades, thus defining the blades, the walls of thehub, and the floor of the air passage at the base of the hub. With theseinserts in place as shown in FIG. 7, molten wax is poured into the die.The wax is allowed to cool and the individual inserts 20 areautomatically extracted radially as shown in FIG. 8 or along some simpleor compound curve as shown in FIGS. 9 and 10 in order to expose thesolid wax pattern 21 and make possible the removal of the pattern fromthe die. FIGS. 7 and 8 illustrate radial extraction, FIGS. 9 and 10 incomparison illustrate extraction along a simple curve, using offset arms22.

[0080] FIGS. 7-10 show 6 dies and 6 blades for ease of illustration;however, as discussed above, the die preferably has a total of either 12(simple) or 24 (compound) inserts for making a total of 6 full lengthand 6 “splitter” blades. As discussed above, in the case of 24 compoundinserts, one set of 12 corresponding inserts is first extractedsimultaneously, and then the second set of 12 corresponding inserts isextracted simultaneously. Compound die inserts can be produced bydividing the air cavity into two sections, and either die insert can beextracted radially or along a curve, depending upon blade design.

[0081] The wax casting process according to the invention occurs fullyautomatically. The inserts are assembled to form a mold, wax isinjected, and the inserts are timed by a mechanism to retract in unison.

[0082] Once the wax pattern (with pour funnel) is formed, the ceramicmold forming process and the titanium casting process are carried out inconventional manner. The wax pattern with pour funnel is dipped into aceramic slurry, removed from the slurry and coated with sand orvermiculite to form a ceramic layer on the wax pattern. The layer isdried, and the dipping, sanding and drying operations are repeatedseveral times to create a multiple layer ceramic shell mold enclosing orencapsulating the combined wax pattern. The shell mold and wax patternswith pour funnel are then placed within a kiln and fired to remove thewax and harden the ceramic shell mold with pour funnel.

[0083] Molten titanium is poured into the shell mold, and after thetitanium hardens, the shell mold is removed by destroying the mold toform a light weight, precision cast compressor wheel capable ofwithstanding high RPM and high temperatures.

[0084] The titanium compressor wheel of the present invention has adesign lending itself to being produced in a simplified, highlyautomated process. As a result, the compressor wheel is not liable toany deformities as might result when using an elastic deformable mold,or when assembling separate blades onto a hub, according to theprocedures of the prior art.

[0085] Tested against an aluminum compressor wheels of similar design,the aluminum compressor wheel as not capable of withstanding repeatedexposure to higher pressure ratios, while the titanium compressor wheelshowed no signs of fatigue even when run through thirteen or more timesthe number of operating cycles as the aluminum compressor wheel.

[0086] Although this invention has been described in its preferred formwith a certain degree of particularity with respect to a titaniumcompressor wheel, it is understood that the present disclosure of thepreferred form has been made only by way of example and that numerouschanges in the details of structures and the composition of thecombination may be resorted to without departing from the spirit andscope of the invention.

[0087]FIG. 11 shows a compressor wheel which corresponds essentially tothe compressor wheel of FIG. 2, except that a modest amount of backsweepis provided at the trailing edge 16 of the blade. This small amount ofbacksweep, taken with the forward rake along the leading edge of theblade, might make it difficult to easily extract a single die insertdefining an entire air passage. To facilitate die insert removal, thecompressor wheel shown in FIG. 11 can be produced using compound dieinserts, i.e., a first die insert for defining the initial or inlet areaof the air passage, and a second die insert for defining the remainingair passage area. The manner in which the air passage is divided intotwo areas is not particularly critical, it is merely important that thefirst and second die insert can be withdrawn either simultaneously orsequentially.

[0088] Although a cast titanium compressor wheel has been describedherein with great detail with respect to an embodiment suitable for theautomobile or truck industry, it will be readily apparent that thecompressor wheel and the process for production thereof are suitable foruse in a number of other applications, such as fuel cell poweredvehicles. Although this invention has been described in its preferredform with a certain of particularity with respect to an automotiveinternal combustion compressor wheel, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of structures and thecomposition of the combination may be resorted to without departing fromthe spirit and scope of the invention.

[0089] Now that the invention has been described,

I claim:
 1. An air boost device comprising: a compressor housing havingan air inlet and an air outlet; and a compressor wheel mounted forrotation within said compressor housing, wherein said compressor wheelis a titanium centrifugal compressor wheel including: a hub defining anaxis of rotation, and a plurality of backswept aerodynamic bladescarried on the surface of said hub and defining air passages betweenadjacent blades, wherein each of said air passages is definable by fromone to three solid die inserts which can be inserted between and pulledfrom between said blades without deformation of said dies or blades. 2.An air boost device as in claim 1, wherein said compressor wheel is acentrifugal compressor wheel adapted for drawing air in axially,accelerating said air centrifugally, and discharging air radially.
 3. Anair boost device as in claim 1, wherein said compressor housing includesa volute-shaped chamber adapted for receiving air discharged from saidcompressor wheel.
 4. An air boost device as in claim 1, wherein thenumber of die inserts necessary to define the air passage between saidblades is three.
 5. An air boost device as in claim 1, wherein thenumber of die inserts necessary to define the air passage between saidblades is two.
 6. An air boost device as in claim 1, wherein the numberof die inserts necessary to define the air passage between said bladesis one.
 7. An air boost device as in claim 1, wherein said compressorwheel is comprised of a titanium alloy.
 8. An air boost device as inclaim 1, wherein said compressor wheel aerodynamic blades comprisealternating full blades (4) and splitter blades (5).
 9. An air boostdevice as in claim 1, wherein said compressor wheel is comprised of atitanium alloy comprising titanium, aluminum and vanadium.
 10. Aturbocharger comprising: a turbine housing including an exhaust gasinlet and an exhaust gas outlet; a turbine wheel rotationally mountedwithin said turbine housing; a compressor housing including an air inletand an air outlet; and a titanium centrifugal compressor wheelrotationally driven by said turbine wheel, wherein said titaniumcentrifugal compressor wheel comprises: a hub defining an axis ofrotation, and a plurality of backswept aerodynamic blades carried on thesurface of said hub and defining air passages between adjacent blades,wherein each of said air passages is definable by from one to threesolid die inserts which can be inserted between and pulled from betweensaid blades without deformation of said dies or blades.
 11. Aturbocharger as in claim 10, wherein said compressor wheel is acentrifugal compressor wheel adapted for drawing air in axially,accelerating said air centrifugally, and discharging air radially.
 12. Aturbocharger as in claim 10, wherein said compressor housing includes avolute-shaped chamber adapted for receiving air discharged from saidcompressor wheel.
 13. A turbocharger as in claim 10, wherein the numberof die inserts necessary to define the air passage between said bladesis three.
 14. A turbocharger as in claim 10, wherein the number of dieinserts necessary to define the air passage between said blades is two.15. A turbocharger as in claim 10, wherein the number of die insertsnecessary to define the air passage between said blades is one.
 16. Aturbocharger as in claim 10, wherein said compressor wheel is comprisedof a titanium alloy.
 17. A turbocharger as in claim 10, wherein saidcompressor wheel aerodynamic blades comprise alternating full blades (4)and splitter blades (5).
 18. A turbocharger as in claim 10, wherein saidcompressor wheel is comprised of a titanium alloy comprising titanium,aluminum and vanadium.
 19. A method as in claim 18, wherein saidtitanium alloy comprises 85-95% titanium, 2-8% aluminum, and 2-6%vanadium.
 20. A method as in claim 18, wherein said titanium alloycomprises approximately 90% titanium, 6% aluminum, and 4% vanadium.